1. Field of the Invention
Aspects of the present invention relate to an organic light emitting diode (OLED) display device and a method of fabricating the same and, more particularly, to an OLED display device which includes a compensation circuit capable of compensating for a threshold voltage of a driving transistor, and can decrease the number of process operations, and minimize a decrease in aperture ratio, and a method of fabricating the OLED display device.
2. Description of the Related Art
A flat panel display device (FPD) has become strongly relied upon as a display device that has superseded a cathode-ray tube (CRT) display device because the FPD is fabricated to be lightweight and thin. Typical examples of the FPD are a liquid crystal display (LCD) device and an organic light emitting diode (OLED) display device. As compared to the LCD, the OLED display device has a higher luminance, a wider viewing angle, and can be made thinner because the OLED display device needs no backlight.
In the OLED display device, electrons and holes are injected into an organic thin layer through a cathode and an anode and recombine to generate excitons. The electrons and holes emit light of a certain wavelength as the electrons and holes recombine.
The OLED display device may be classified into a passive matrix type and an active matrix type depending upon how the device drives N×M pixels that are arranged in a matrix shape. An active matrix type OLED display device includes a circuit using a thin film transistor (TFT). A passive matrix type OLED display device can be fabricated by a simple process since anodes and cathodes are arranged in a matrix shape on a display region. However, the passive matrix type OLED display device is applied only to low-resolution, small-sized display devices because of the resolution limit, high driving voltage, and short lifetimes of materials. By comparison, in the active matrix type OLED display device, a TFT is mounted in each pixel of a display region. Thus, a constant amount of current can be supplied to each pixel so that the active matrix type OLED display device can emit light with a stable luminance. Also, since the active matrix type OLED display device consumes less power, the active matrix type OLED display device can be applied to high-resolution, large-sized display devices.
In an active matrix type OLED display device, a threshold voltage of a driving transistor included in each pixel has an inconstant deviation due to problems in the fabrication of a TFT. Since the inconstant deviation of the threshold voltage makes the luminance of the OLED display device nonuniform, the OLED display device needs to include a pixel circuit having a variety of compensation circuits in order to compensate for such inconstant deviation of the threshold voltage.
However, the pixel circuit of the OLED display device further includes a plurality of TFTs and at least one capacitor in order to compensate for the deviation of the threshold voltage of the driving transistor. As a result, the pixel circuit has a complicated configuration, thus degrading reliability and complicating fabrication processes.